The present invention relates to a lens meter having a focusing indication system with divided-image registration focusing as an auxiliary means for focusing determination.
There have heretofore been available two methods for visual judgement of focusing in lens meters. One of the methods utilizes the sharpness of an image as information for focusing determination and has found wide use in many lens meters. The advantage of this method is that since a lens being examined is measured substantially through the entire area of a lens mount opening, it is relatively easy to find various faults such as polishing defects and striae of the lens. However, the method is disadvantageous in that as the power of a single lens (usually a spectable lens) increases, aberrations are increased to the point where focusing determination is extremely difficult to make, and thus it is time-consuming to effect focusing and any focused condition cannot be reproduced easily.
The other method relies on separate-image registration focusing as disclosed in Japanese Patent Publications Nos. 42-12107 and 42-14269. This method relies on the fact that the vernier acuity of human vision is extremely good, and allows focusing to be effected in a short period of time with improved focusing reproducibility. However, the method as disclosed in the foregoing publications suffers from many shortcomings. A first difficulty is that any polishing faults and striae of a lens being examined are difficult to find, as a narrow beam of light is employed for examination. A second problem is that a bright light source is required to compensate for any brightness shortage due to the narrow light beam used. A third difficulty is diffraction causing unsharp images, though the depth of focus is large due to the narrow light beam. A fourth shortcoming is that both of the apparatus disclosed in the above publications are unfamiliar to those who have been accustomed to the measurement process by the conventional lens meters. A fifth defect is that the disclosed apparatus have complex optical systems which involve many difficulties in the manufacture and adjustment of the apparatus.